1. Field of the Invention
The invention involves thermal regulation systems for controlling the temperature of an environment, and the thermostat systems used in thermal regulation systems. The invention, in particular, solves long standing problems associated with the use of a compact creep action bi-metal thermostat as a feedback device for monitoring and controlling the thermal conditions in a regulated environment. The invention has particular utility when it is used to control the temperature of a temporary work environment such as a building construction site.
2. Description of the Prior Art
Thermal regulation systems involve two principal components, a thermal regulator for altering the temperature of an environment and a thermostat for determining the temperature of the environment and for actuating the thermal regulator as necessary to maintain the temperature of the environment. Typically a thermal switch is used to actuate the thermal regulators, and thus, to maintain the thermal conditions in the environment. The thermal switch opens or closes an electrical circuit when the temperature of an environment to which it is exposed passes from one range of temperatures and into another range of temperatures. In these systems, the thermal regulator is controlled by the condition of the electrical circuit.
One such thermal switch is the bi-metal thermostat. The bi-metal thermostat works by taking advantage of the principal that dissimilar metals have different coefficients of thermal expansion, and therefore they expand at different rates when exposed to temperature increases. When two dissimilar metals are bound together they form what is known as a bi-metal strip. When a bi-metal strip is exposed to heat, the bi-metal strip begins to expand and lengthen. However, one metal strip of the bi-metal expands faster than the other strip. This causes the bi-metal strip to bend as the temperature to which it is exposed increases. Bi-metal devices use this bending or "creep" action to move an electrical contact element into and out of engagement with another contact element when the bi-metal strip is exposed to an environment having a temperature within certain range. The contacts are connected, in series, with an electrical circuit. The electrical circuit activates or de-activates a thermal regulator, thus altering the temperature of the environment being controlled by the bi-metal thermostat.
The use of the creep action device to control a thermal regulator has not been favored in practice because the creep action of the bi-metal allows small changes in the temperature of the environment to cause separation or engagement of the contact elements. This results in excessive cycling of the thermal regulator. This excessive cycling enhances the wear on the thermal regulator, results in overregulated and unstable conditions in the environment, and causes user dissatisfaction with the thermal control system.
Excessive cycling of devices controlled by bi-metal thermostats is also caused by the fast response of the creep action bi-metal thermostat to changes in the temperature of the environment. This fast response leads to irregular performance as it causes the bi-metal thermostat to react to local or transient changes in the temperature of the environment. This may result in the unwarranted actuation of the thermal regulator and improper modulation of the temperature of the environment. This also may cause the thermal regulator to execute short cycle engagements increasing the wear and tear on the thermal regulator and reducing the life cycle of the components of the thermal regulator.
The limitations of the creep action bi-metal thermostat have led to the development of the snap action bi-metal thermostat. The snap action bi-metal thermostat also uses a bi-metal device to bring electrical contact elements into and out of contact with one another. However, it differs from the creep action bi-metal thermostat in that a magnet is inserted into the system to cause a change in the engagement cycle of the contact elements.
In one embodiment of the snap action thermostat, two metallic strips are used. One of these strips is a bi-metal strip and the other strip is a single metal strip. One end of each strip is fixed, and the strips are positioned in spaced relation to each other. An electrical contact element is fixed to the other end of the bi-metal strip and the bi-metal strip is permitted to bend in response to temperature changes. Another electrical contact element is fixed at the other end of the single metal strip. However, the other end of the single metal strip is positioned so that the creep of the bi-metal strip will move the contact element at the end of the bi-metal strip into engagement with the contact element of the single metal strip when the bi-metal thermostat is exposed to an environment having a temperature within a certain range.
These strips are then electrically connected, in series, to an electric circuit. Thus, when the environment to which the bi-metal strip is exposed remains within one range of temperatures, the contacts at the ends of the elongated strips do not contact each other and the electric circuit is left open. At other temperatures the contacts are engaged, and the electrical circuit is completed.
This action is consistent with the action of a standard creep action bi-metal thermostat. However, in a snap action thermostat, a magnet is positioned to urge the contact element on the bi-metal strip into engagement with the contact element on the single metal strip. Thus, when the contact element on the bi-metal strip approaches the contact element on the single metal strip, it reaches a certain position where magnet exerts sufficient force to pull or "snap" the contact elements into engagement. At this point, the circuit to which the thermostat is connected is completed causing an appropriate reaction from the thermal regulator.
When a snap action thermostat is used in conjunction with a cooling system, the snap action causes the contacts to engage at temperatures above the temperatures required to cause the contacts of the creep action bi-metal thermostat to engage. Further, the magnetic attraction will tend to hold the contacts in an engaged position at temperatures that would cause the contacts of a creep action bi-metal thermostat to separate. Thus, the snap action thermostat introduces a thermal differential or a range of temperatures within which the thermal conditions of the environment may deviate from the preferred temperature. By allowing the temperature of the regulated environment to vary within this range, short cycling of the thermal regulator is avoided.
The use of a relatively expensive bi-metal snap acting thermostat to detect temperature changes in an environment and to provide a signal change when the temperature of the environment passes above or below the differential is well known. Thermostats built upon snap action technology are generally of the adjustable type, and relatively expensive. They are also large in size and must typically be mounted on a wall of other structures. While non-adjustable, creep action, bi-metal thermostats are less expensive, more rugged and substantially smaller, the absence of a thermal differential has limited their use.
The problems associated with the absence of the thermal differential in a creep action bi-metal thermostat are particularly pronounced in circumstances where a creep action bi-metal thermostat is called upon to regulate the temperature of an environment that must be maintained with tight limits. In such circumstances, the contacts are maintained within close proximity to each other at all times arid, accordingly, small changes in the thermal conditions to which the bi-metal thermostat is exposed may actuate the thermal regulator. Under such circumstances, the creep action bi-metal thermostat is particularly vulnerable to local and transient temperature variations.
Accordingly, it is an object of this invention to provide a thermal regulation system and thermostat system using a creep action bi-metal thermostat design that does not execute short cycles caused by the absence of a thermal differential.
The creep action bi-metal thermostat has also been found to be subject to a second design limitation. In creep action bi-metal thermostats, the contact element on the bi-metal strip is brought into and out of engagement with the contact element on the single metal strip due to the thermal expansion of the bi-metal strips. At certain temperatures, these contact elements may be held in very close proximity to each other without actually making contact. Because the contact elements are electrically connected in series with an electrical circuit, a difference of potential exists between the contact elements. Under proper conditions, this difference in potential may cause arcing between the contact elements. When this occurs, heat is released. This heat affects the bi-metal strip causing an expansion of the strip which can result in untimely disengagement of the contact elements.
This arc heating problem can have serious consequences when the device is used as a feedback means in a closely regulated system. Because the internal temperature of the bi-metal thermostat is raised by exposure to the arc, the bi-metal thermostat may actually disengage a heating system only seconds after this heating system has been engaged. Conversely, arc heating may cause a cooling system to stay on longer than necessary. This results in improper performance of the bi-metal thermostat and irregular performance of thermal regulation systems that rely upon the bi-metal thermostat.
In certain applications the use of a shorter or more compact thermostat is an advantage. Such a compact or "short strip" creep action bi-metal thermostat is particularly vulnerable to the effects of arc heating because of the reduced mass of the bi-metal strip. Unfortunately, no solution exists in the art that will protect such "short strip" thermostats from irregular performance caused by arc heating.
Thus, it is a further object of this invention to provide a thermal regulation system and a thermostat system using a creep action bi-metal thermostat that is protected from short cycling caused by the absence of a thermal differential and also protected from short cycling caused by the heat generated by arcing between the contacts.
The need for this invention is particularly acute in conjunction with thermal regulation systems used at building construction sites. Typically, if workers at construction sites are provided with thermostats having adjustable settings, they will set these thermostats systems at maximum hot or maximum cool settings. This results in additional expense in unnecessary heating and cooling. However, the installation of more expensive complicated thermostats at building construction sites has not proven to be successful because of the expense and delay associated with the installation of such thermostats. Further, such systems are not compact in size and thus they are vulnerable to damage at construction sites. The obvious value of these devices also makes them an attractive target for vandals and thieves.
However, it is not clear that adjustable thermostats are necessary at building construction sites. Rather, experience teaches that the optimal working temperature of a construction environment is relatively constant, between 60 and 70 degrees Fahrenheit. Thus, in construction sites, the use of an adjustable thermostat is not absolutely necessary. Rather, single set point thermostats can be used. The use of a compact single set point or non-adjustable creep action bi-metal thermostat to monitor the work environment is particularly advantageous. Such a thermostat would reduce energy costs, yet be inexpensive. Further, such a thermostat is easily installed, difficult to damage, unlikely to be stolen, and easily removed.
Accordingly, it is a further object of this invention to provide a compact low cost single set point creep action bi-metal thermostat system for temporary use at construction sites, or in other thermal regulation systems where the cost of the components, particularly the thermostat, must be closely limited, and where the thermostat must, respond to temperature variations, within a tightly defined range.